An electric traction vehicle is a vehicle that uses electricity in some form or another to provide all or part of the propulsion of the vehicle. This electricity can come from a variety of sources, such as stored energy devices relying on chemical conversions (batteries) to create electrical energy, stored energy devices relying on stored electrical charge (capacitors), stored energy devices relying on mechanical stored energy (e.g., flywheels, pressure accumulators), and energy conversion products. In a typical conventional electric traction vehicle, a prime mover, such as a diesel engine, is used to drive an electric generator or alternator which supplies electric current to one or more traction motors. The traction motors typically are coupled to wheel sets on the vehicle. A typical vehicle that utilizes this type of electric traction is a railroad locomotive. In some conventional electric traction vehicles, stored energy is used to provide the main power which provides the electrical current to one or a plurality of traction motors. A typical vehicle that utilizes this type of electric traction is a golf cart or battery powered electric car. In some conventional electric traction vehicles, having more than one source of energy is desirable, such as a stored energy unit and an internal combustion engine coupled to a generator. By having more than one source of energy, some optimizations in the design can allow for more efficient power production, thus allowing power to be used from different sources to come up with a more efficient system for traction. These types of vehicles are commonly referred to as hybrid electric vehicles (HEV). Series and Parallel HEV system designs are what is usually encountered.
In a typical electric vehicle, regenerative braking capability may be provided in addition to or instead of a mechanical braking system by configuring the electric traction motors to function as generators rather than motors, such that the flow of electrical power to the electric traction motors is reversed. In this regeneration mode, each of the electrical traction motors receives mechanical energy from the rotation of the wheel set coupled to the traction motor and operates as a generator to convert the mechanical energy to electrical energy and provide an electrical power output. This process of receiving mechanical energy from the wheels to operate the electrical traction motor as a generator absorbs mechanical energy and acts to reduce the speed of the vehicle. Additionally required braking force is typically provided by a mechanical brake.
The reverse flow of electrical power from the electrical traction motors is typically diverted to the stored energy device. If the diverted electrical power exceeds the capacity of the stored energy device, the excess electrical energy is typically diverted to an energy dissipation device or dynamic brake, such as a resistive element, and is dissipated as heat through the resistive element. Storing and dissipating excess electrical energy in this manner over a large number of braking cycles may decrease the useful life of the stored energy device, and the use of an additional energy dissipation system adds cost and complexity to the system depending on the required size. Further, high costs may be associated with using and maintaining a mechanical braking system, particularly on heavy-duty electric vehicles, such as fire trucks, military vehicles, refuse-handling vehicles, etc. Thus, there is need for a system and method for braking in an electric vehicle which utilizes an engine and generator combination to configured to provide additional braking capability and to dissipate excess energy during regenerative braking.